Equation (REF ) for the second moments
includes first-order spatial differential
operator \(\hat{\cal M}\)  applied to the
third-order moments \(\Theta ^{\rm (III)}\) . The problem arises
how to close equation (REF ), i.e., how to express
the third-order term \(\hat{\cal M} \Theta ^{\rm (III)}\) 
through the lower moments [1]}, [2]}, [3]}, [4]}.
We use the spectral \(\tau \)  approach which
is a universal tool in turbulent transport
for strongly nonlinear systems.
The spectral \(\tau \)  approximation
postulates that the deviations of the third-moment terms,
\(\hat{\cal M} \Theta ^{\rm (III)}({k})\) , from the contributions to
these terms afforded by the background
turbulence, \(\hat{\cal M} \Theta ^{\rm (III,0)}({k})\) ,
can be expressed through similar deviations
of the second moments, \(\Theta ^{\rm (II)}({k}) -\Theta ^{\rm (II,0)}({k})\)  as [4]}, [6]}, [7]}
\(&& \hat{\cal M} \Theta ^{\rm (III)}({k}) - \hat{\cal M}\Theta ^{\rm (III,0)}({k})= - {\Theta ^{\rm (II)}({k}) - \Theta ^{\rm (II,0)}({k}) \over \tau _r(k)} ,\nonumber \\\) 
